The effects of impurity elements on the reduction of wustite and magnetite to iron in CO/CO 2 and H 2 /H 2 O gas mixture
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I.
INTRODUCTION
IN commercial
practice, t h e treatment of iron ores by sintering or pelletizing processes produces complex microstructures containing primary and secondary iron oxides, iron-containing compounds, and glassesJ 1,21The chemical compositions of the primary and secondary iron oxides are often considerably different from the original raw materials and contain impurity elements in solid solution introduced by reaction with surrounding phases. Previous wo rk er s [3-16] have shown that the impurities dissolved in the solid iron oxides can have significant effects on their reduction behavior, affecting the microstructural changes on reduction, the pore size distribution, and the overall reduction kinetics. In the present study, the effects of impurities on the reaction mechanisms which occur during the final stages of reduction, the transformation of magnetite and wustite to iron, are examined, and the critical H2/H20 and CO/CO2. mixtures required for the production of a porous iron product morphology are determined. II.
EXPERIMENTAL
The methods of preparation of the wustite and magnetite samples and the reduction techniques used have been described in previous publications. [17,181 One-mm 3 dense wustite samples were prepared by the addition of known quantities of impurity oxides to spectrographically pure iron powder. The mixtures were pressed into 12-mm-diameter pellets, and these were sintered in hydrogen at 1250 K for 4 days. The samples were cooled
and repressed before oxidation in 50 pet CO, 50 pet C02, at 1400 K for 2 days. For samples containing phosphorus, presintering was carried out at 773 K. Sodium and potassium additions were in the form of the carbonates, and the pressed iron pellets were sintered directly in 65 pet CO, 35 pet CO2 at 1100 K for 3 days to prevent excessive vaporization of potassium. One-mm 3 specimens for reduction were then cut from the pellets using a scalpel. Analyses of the impurity levels in solid solution in the unreduced wustite samples were carried out using the electron microprobe (Tables I through III). In the cases of Si, Na, and K, the maximum solubility of the elements in wustite was found to be very low, and these samples having single-element additions can be considered to be saturated with those elements. While not all of the other impurities are present at their limit of solubility, the range of compositions chosen for each element covers the concentrations most likely to be encountered in commercial practice. It should also be noted that the wustite stoichiometry will vary with impurity content, temperature, and gas composition; however, the authors are not aware of any studies accurately defining these effects. Examination of the partially reduced samples was carried out using both optical and scanning electron microscopy. III.
EXPERIMENTAL RESULTS
Experiments reported in this investigation involve reduction in CO/CO2 and Hz/HzO gas mixtures at a total pressure of 1 atm at temperatures in the range of 1073 to 1373 K. A. Single-Element Additions to W
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